1. Technical Field
The invention relates to a method and apparatus for tracing a path traversed by an entity or object, for example an article, e.g. a container shipment, asset or person, such as a person under surveillance, potentially worldwide. The invention also relates to a tag for use with the method and a method of providing a historical database of frequency spectra extant at different geographical locations.
2. Background Art
The popularity and availability of the Global Positioning System (GPS) and low cost radio frequency identification (RFID) systems have led to significant increases in the use of systems for tracking articles or people. Thus, GPS is becoming a commodity product for the transport industry and RF (Radio Frequency) tags are also becoming widely used for indoor tracking of articles and for access control.
A key feature of these tracking systems is their ability to provide for accurate real-time location of the article under observation. To do so, however, they employ devices which must either be active (transmit RF signals) or rely upon other RF transmitters (e.g. GPS or WAN system) with known positions.
For example, U.S. Pat. No. 5,014,040 and, more recently, U.S. Pat. Nos. 6,891,469 and 6,889,135, disclose tracking systems in which an active radio transmitter associated with the article or person being tracked is used to set up a known radiation field, and multiple tuned receivers are used to detect the presence of the radio transmitter, the location of the transmitter being determined using triangulation. Disadvantages of such active RF systems include short operating times for transmitters or transceivers that, as a general rule, will be battery operated, and the fact that the transmissions may be detected easily, making them unsuitable for covert operation.
Some known systems utilize existing navigational systems, such as LORAN and Global Positioning Systems (GPS) to determine the exact location of the vehicle, in terms of geographic coordinates (longitude and latitude), either during its travel or at the locations of specific operations such as loading or unloading. Examples are disclosed in U.S. Pat. No. 5,040,240 (Keegan) and U.S. Pat. No. 5,014,206 (Scribner et al.).
U.S. Pat. Nos. 6,606,556 and 5,223,844 disclose security systems based upon GPS technology, the former for tracking a person, and the latter for tracking a stolen vehicle. A major disadvantage of such GPS-based tracking systems is that they require a line-of-sight between the GPS antenna and the satellite(s). In addition, the receiver circuitry is too large to meet most size requirements of covert systems and power consumption may limit battery life.
U.S. Pat. No. 6,774,797 discloses a system for tracking movements of an individual by means of a tracking tag which uses GPS and/or cellular telephone geolocation methods to determine its location and transmits the location information to a monitoring center.
U.S. Pat. No. 5,648,763 discloses a security system for controlling access to cargo containers which has a receiver for receiving LORAN-C or GPS signals and using them to produce a position signal corresponding to the current geographic position of the container. This position signal is compared with a preset signal representing a predetermined geographic position and, when the two correspond, an enabling signal permits the security system to gain access to the container. Again, the use of GPS introduces line-of-sight limitations, LORAN-C is limited to maritime applications, and only the one location of the container is of interest or consequence.
There is also a growing need for tracing systems which differ from tracking systems because, at least in the context of this specification, a tracking system is defined as a system that allows determination of the current geographical location of the article being tracked, whereas a tracing system is defined as a system that allows determination of a geographical path traversed by the article after it has completed its travels.
Some known systems, specifically systems using active RF techniques, provide tracking and/or tracing capabilities but, as compared with passive RF techniques, have limitations in that they do not provide, for example, (i) very low DC power consumption, meaning longer operational life; (ii) simple electronic circuitry, leading to miniature dimensions and very low cost; and (iii) inaccessible real-time data, meaning better compliance with human rights legislation.
U.S. Pat. No. 6,480,108 discloses a system for tracing the path taken by a tag through several defined geographic areas. The tag includes a transmitter and a receiver. Tag readers in the different geographic areas transmit RF signals and, in response, the tag transmits RF signals to the tag-readers. Knowing the areas in which the tag readers are located, the system can trace the path taken by the tag. This is an active system and so suffers from at least some of the disadvantages of active tracking systems, as described above.
It is also known to use signals from cellular telephone base stations to track vehicles. For example, U.S. Pat. No. 5,895,436 discloses a system for tracking a vehicle using cellular telephone signals which are detected by a cellular transceiver installed in the vehicle. Such cellular telephone tracking systems suffer from lack of ground coverage and dead spots, especially when the vehicle is indoors. Moreover, like GPS receives, cellular telephone receivers consume substantially more DC power in comparison to FM receivers, especially if the FM receiver is used without any audio output.
It is also known to use electronic identification tags to track the movement of livestock For example, U.S. Pat. No. 6,577,241 discloses a system in which a manufacturer creates a plurality of identification tags each having a unique identification number that is associated with a corresponding “prerecord” in a central database. Each customer purchasing a tag is given access to the database to enter or read data in the corresponding prerecord. Although the system allows the movement of the animal to be traced, the tag is not neither a radio transmitter nor a radio receiver. The system requires data to be entered and updated by the customer.
U.S. Pat. No. 5,939,982 discloses a system for detecting opening of a shipping container which houses, along with the goods-in-transit, at least the antenna of an FM receiver that monitors FM broadcast signals. If the receiver detects an increase in the received signal strength, evidencing at least partial or opening of the container, it produces an alarm signal. The receiver may also scan and store the FM spectrum pertaining at the time the container was open. Comparison of the stored FM spectrum with charts of FM spectra for various cities allows an investigator to identify the approximate location of the container when the intrusion occurred. While this arrangement maybe acceptable in the context of intrusion detection, the receiver only scans and stores location information if the container is opened, and the system does not provide for tracing a path taken by the container. Also, the comparison is not automatic.